The spectral characteristics of four different types of apodized fiber Bragg gratings with a single π phase shift are analyzed based on the simulation. The 2-discrete Gaussian apodization is proved to have the most effective suppression to grating side mode. A novel asymmetrical distributed feedback fiber laser based on this apodization structure is presented and fabricated as well. The grating has a −20 dB side-mode suppression. The laser exhibits a high power ratio of backward to forward outputs. It has a relative intensity noise of −90dB/Hz and a linewidth of 20k Hz operating in a single polarization longitudinal mode.
© 2013 OSA
Distributed feedback fiber laser (DFB-FL) operating in 1.55μm optical communication band has been extensively studied and developed for its unique characteristics such as narrow spectral linewidth, robust single longitudinal mode operation and compact dimension since its first invention [1–11]. And as sensing element it shows ultrahigh sensitivity in acoustic pressure sensing, and great potential in large scaled wavelength division multiplexing [5,7,8]. Some demonstrated sensing arrays with up to 16 elements have been reported . The applicable number of serially wavelength division multiplexed sensors is strongly dependent on the grating performance. The grating reflection bandwidth is a key limiting factor because the finite bandwidth in optical frequency domain will limit the minimum wavelength spacing between lasers.
To achieve the high performance such as narrow bandwidth with high side-mode suppression several apodization methods have been presented and utilized [2,7,8]. In 1999, D. Y. Stepanov  reported an apodized DFB-FL with enhanced side-mode suppression by utilizing a super-Gaussian apodization for the first time. In 2004, K. Yelen  presented a step apodized DFB-FL with the improved efficiency. In 2007, A. Tikhomirov  theoretically and experimentally demonstrated the high side-lobe suppression and negligible cross coupling for DFB-FL by incorporating a distributed phase shift and amplitude-apodized grating design. In 2008, a review about DFB-FL strain sensors was given by G. A. Cranch . In that paper the double apodized phase-shift grating was introduced and the equivalent reduction on side-lobe level in phase-shifted grating could be achieved compared with no phase-shifted grating.
In this paper we systematically investigate the side-mode suppression for phase-shifted gratings with four specific apodization profiles. An asymmetrical DFB-FL grating with the 2-discrete Gaussian apodization and single π phase-shifted structure is presented and fabricated. The power performances and laser characteristics are investigated experimentally. The results show that the proposed apodization could induce high side-mode suppression for grating and highly unidirectional outputs while it is easy to be fabricated. Therefore, it is believed that this type of apodized DFB-FL is significant and useful for the sensing applications in dense wavelength division multiplexed configuration.
2. Simulation and analysis
Based on the transfer matrix method we carry on the simulation of several types of typical apodizations with single discrete π phase shift. The apodization profiles and resulted grating spectra are compared. For a constant grating length of 5.5cm the supposed coupling strength KL (K is the coupling coefficient and L is the grating length.) is set to 7.5 to get the approximately maximum output and the peak coupling coefficients are 133cm−1, 246cm−1, 215cm−1, 236cm−1, and 205cm−1 for uniform, single Gaussian, single Super Gaussian, compact double Gaussian, and 2-discrete Gaussian apodization, respectively. From the simulation results shown in Fig. 1 we can see that the four apodization examples all have no narrowing effect on the main reflection band. Only the 2-discrete Guassian apodization gives as narrow main reflection band as the uniform grating. The single Gaussian and super-Gaussian apodizations indeed don’t have any effect on reducing the side band reflection while they only smooth the reflection spectra. For the example with a compact double Gaussian apodization profile which is proposed in , the first order side lobe is significantly suppressed by −15dB while the other high order side lobes are only suppressed by −10dB compared with the uniform single π phase shift grating. The 2-discrete Gaussian apodization has an evident suppression on the over-second order side lobes where it shows an over −25dB suppression on the second order side lobe and almost a −40dB suppression on side lobe at a 1nm bandwidth compared with the uniform case though there is not any suppression on the first order side lobe.
Therefore in general it is believed that the 2-discrete Gaussian apodization gives more effective suppression on side lobes without broadening the main reflection band in the phase-shifted grating. Indeed this presented apodization phase-shifted grating is composed of two discrete apodized grating separated by a single discrete phase shift from each other. This structure is also easy to be fabricated in comparison to that with distributed phase shift . For fully utilizing the output laser and reducing the influence to the adjacent elements in multiplexed sensing application the unidirectional output is always desired. We here introduce an asymmetrical amplitude apodization to DFB-FL which is displayed in Fig. 2(a). The output ratio could be theoretically obtained by output simulation using transfer matrix method or calculating the transmission of the two subsection grating separated by phase shift. For example, in this work the transmission of the backward and forward sub-gratings are 8.3% and 0.68% where the ratio is 12. Similarly the enhanced side-lobe suppression is achieved in comparison to the other DFB structures. The calculated laser threshold for 2-discrete Gaussian apodized and uniform DFB-FLs are shown in Fig. 2(b). It is evident that the high order thresholds are strongly enhanced for the 2-discrete Gaussian apodization structure. The robust single longitudinal mode operation will be achieved.
A 5.5 cm long phase-shift grating was written in a 6 cm long photosensitive Erbium doped fiber (Nufern, peak absorption at 1530 nm is 8 dB/m) which was spliced to the passive matching pigtail fiber (Nufern 980Hp) at both ends. The grating was written by scanning 244 nm frequency-doubled harmonic Argon ion continuous wave laser across the phase mask and fiber. A polarization dependent grating was generated by the vertically polarized UV scanning laser. The DFB-FL configuration and the fabrication setup diagram are shown in Fig. 3. The method used here is similar to the method described in  and . The apodization for grating is achieved by dithering phase mask to change the visibility of the interference fringe at fiber while the phase shift is introduced by a simple displacement of the phase mask to fiber during the beam scanning. The displacement of the phase mask for an accurate π phase-shift is a quarter of the phase mask period while the phase mask is mounted on a piezoelectric transducer (PZT) stage (PI, P-752.11c) with nanometer resolution.
The spectrum of the fabricated grating measured with an optical spectrum analyzer (OSA, Yokogawa AQ6370C, 0.01nm wavelength resolution) is shown in Fig. 4. The first order side lobe and the phase-shift transmission slit cannot be distinguished for the resolution limit of the OSA. Just as the above simulation the suppression for the high order side lobes is significantly enhanced. The highest visible side lobe is −22dB from the reflection peak while the side lobe at 1nm bandwidth is −25dB below. The suppression effect is not as good as the simulation but already much better than the uniform grating. We think the even higher side-lobe suppression and unidirectional degree for outputs can be achieved after further optimization.
4. Results and discussion
The fiber laser performances of the fabricated DFB-FL including output powers, laser spectrum, relative intensity noise (RIN), laser linewidth, and polarization state are systematically investigated. The results are shown in Fig. 5 and Fig. 6, respectively. We can see that this DFB-FL has a very low threshold about 1mW and a high ratio of the backward to forward output powers about 20:1 which is greater than the calculation. From the laser spectrum measured with an OSA (Apex 2040) having ultrahigh resolution of 0.16pm the DFB-FL is found to operate in a single longitudinal mode. Also we measured its polarization characteristics with a polarization analyzer (Agilent N7788B). The degree of polarization (DOP) is equal to 1 with the stable state of polarization (SOP) which indicates the single polarization operation of the DFB-FL. In Fig. 6(a) the measured RIN spectrum of the DFB-FL at 60mW 980nm pump power shows that it has an evident relaxation oscillation peak of −90dB/Hz at 260kHz. The mean RIN level is below −100dB/Hz within the measured frequency range up to 1MHz. The other peak around 5 kHz in the spectrum has been testified to be caused by the RIN of the used pump. For measuring the linewidth a self-homodyne method employing a 30km fiber delay line is used. It is about 400 kHz with −20dB from the peak, which indicates a 20 kHz FWHM line-width of the measured laser at 60mW pump.
Compared with the previously demonstrated results by  and  this work combine the benefits of narrower main reflection band, higher side-lobe suppression in grating spectrum, asymmetrical laser outputs and single polarization laser operation. The grating structure consisting of 2-discrete Gaussian apodization and a single π phase shift is also easy to be fabricated.
The designed asymmetrical DFB structure gives a high output power ratio of 20:1 which means the more effective utilization of the laser power in sensing application and the reduced influence to the adjacent elements in the serially multiplexed array.
We analyzed the characteristics of the apodized phase-shift gratings with four different types of apodization profiles and single discrete π phase shift. The highest side-lobe suppression could be obtained by the 2-discrete Guassian apodization structure. For the multiplexed sensing application we presented an asymmetrical structure based on this apodization. The presented DFB-FL was fabricated and tested in detail. The high side-lobe suppression about −20dB in grating spectrum and high output power ratio over 20:1 were achieved. It operated in a single polarization longitudinal mode and showed good intensity and spectral characteristics which are −90dB/Hz peak at 260kHz relaxation oscillation frequency and 20kHz FWHM at 60mW pump, respectively. The good performances including high side-lobe suppression, asymmetrical outputs, robust operation in single polarization longitudinal mode with low intensity noise and narrow linewidth will make it very useful and attractive in the large scaled wavelength division multiplexed sensing applications
This work is financially supported by Research Award Fund for outstanding middle-aged and young scientists of Shandong Province of China (BS2010DX030), International Science and Technology Cooperation Program of China (2012DFA10730), and Natural Science Foundation of Shandong Province of China (ZR2010FM039).
References and links
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